The state of polarization of an optical beam can be a significant factor in determining the beam's spatial profile, particularly if the optical beam is focused to a small spot using high numerical aperture optics. For example, if a “donut” shaped focused spot is desired having an on-axis null, beam phase must vary in such a way as to destructively interfere on-axis in order to create the null. However, even if beam phase is precisely controlled, destructive interference may not be complete depending on the beam state of polarization. If the optical beam is tangentially polarized, a deep null can be formed. If the optical beam is radially polarized, the beam will have axial components that do not interfere destructively, and the depth of the beam null will be limited. This is especially true for beams focused to a small spot with high numerical aperture optics, as the magnitudes of the axial polarization components increase as a function of numerical aperture. For applications requiring an optical beam having a deep central null, polarization control may permit increased resolution.
Properties and applications of tangentially and radially polarized beams are described in Toussaint et al., U.S. Pat. No. 7,599,069, which is incorporated herein by reference. According to Toussaint, tangentially polarized beams can be used in microscopy, including multi-photon microscopy as well as other applications. Schultz et al., U.S. Patent Application Publication 2006/0146384A1, which is incorporated herein by reference, describes the use of tangentially and radially polarized beams in microlithography.
Conical surfaces have been used to produce tangentially and radially polarized beams. For example, Schäfer, “On Some Properties of Axicons,” Appl. Phys. B 39:1, 3 (1960), which is incorporated herein by reference, describes a polarizer that uses four conical surfaces to produce a transmitted optical beam that is either tangentially or radially polarized based on the dielectric coating applied to the conical surfaces. Other previous attempts to provide suitable polarization control are described in Lipson et al., U.S. Patent Application Publication 2007/0183036 and Kamon, U.S. Pat. No. 5,436,761, both of which are incorporated herein by reference. Lipson discloses two conical reflectors positioned opposite each other and a polarizer situated between the two conical reflectors. Lipson's conical reflectors reflect input beams in all polarization states, and require a polarizer to provide a polarized output beam.
In order to provide superior performance in multi-photon microscopy, microlithography, and other applications, improved polarizers and polarizing beam splitters are needed.